How to diagnose Atrioventricular Nodal Reentrant Tachycardia (AVNRT)?

Diagnosis of Atrioventricular Nodal Reentrant Tachycardia (AVNRT)

The diagnosis of AVNRT is established by obtaining a 12-lead ECG during tachycardia, which characteristically shows a regular narrow QRS-complex tachycardia (<120 ms) with absent or barely visible P waves that are hidden within or immediately after the QRS complex, creating pathognomonic pseudo r' waves in lead V1 and pseudo S waves in inferior leads (II, III, aVF). 1

Clinical Presentation

The clinical history provides critical diagnostic clues before ECG documentation:

• Palpitations in the neck are highly specific for AVNRT (odds ratio 3.54), distinguishing it from other supraventricular tachycardias 2. These represent regular "cannon A waves" from simultaneous atrial and ventricular contraction against a closed tricuspid valve 3.

• Sudden onset and termination of regular palpitations (paroxysmal supraventricular tachycardia pattern) strongly suggests AVNRT or AVRT 1.

• Response to vagal maneuvers (Valsalva, carotid massage) that terminates the arrhythmia further supports a reentrant tachycardia involving AV nodal tissue 1.

• Polyuria may occur from atrial natriuretic peptide release due to increased atrial pressures 1.

• Female sex and younger age at symptom onset are additional predictors (odds ratios 2.96 and 1.27 respectively) 2.

ECG Diagnostic Criteria During Tachycardia

Whenever possible, obtain a 12-lead ECG during tachycardia (though this should not delay emergency treatment if hemodynamically unstable) 1:

Typical AVNRT Features:

• Narrow QRS complex (<120 ms) with regular RR intervals 1

• Absent or barely visible P waves - atrial activation occurs nearly simultaneously with ventricular activation 1

• Pseudo r' wave in lead V1 - representing the terminal portion of the retrograde P wave deforming the end of the QRS complex 1

• Pseudo S waves in inferior leads (II, III, aVF) - these findings are pathognomonic for AVNRT 1

• Short RP interval - if P waves are visible, they are closer to the preceding QRS than the following QRS 1

Differential Diagnosis Algorithm:

If the tachycardia has a narrow QRS and regular rhythm with no visible P waves, AVNRT is the most common mechanism 1.

If P waves are present in the ST segment separated from the QRS by >70 ms, AVRT is more likely 1.

If RP interval is longer than PR interval, consider atypical AVNRT, permanent form of junctional reciprocating tachycardia (PJRT), or atrial tachycardia 1.

Diagnostic Maneuvers During Tachycardia

Adenosine or carotid massage can aid differential diagnosis - a 12-lead ECG should be recorded during administration to observe the response 1. These maneuvers may terminate AVNRT or reveal underlying atrial activity in other arrhythmias 1.

Esophageal pill electrodes can be helpful if P waves are not visible on surface ECG 1.

Documentation Strategies When Not in Tachycardia

For patients with paroxysmal symptoms but no documented arrhythmia:

• Resting 12-lead ECG should always be obtained, though it is typically normal in AVNRT (unlike AVRT where pre-excitation may be visible) 1

• 24-hour Holter monitoring for frequent episodes (several per week) 1

• Event or loop recorders for less frequent arrhythmias 1

• Implantable loop recorders for rare symptoms (<2 episodes per month) with severe symptoms 1

• Exercise testing if arrhythmia is clearly triggered by exertion 1

When to Proceed Directly to Electrophysiology Study

If clinical history clearly indicates paroxysmal regular palpitations and the resting ECG provides no diagnostic clues, further diagnostic tests for documentation may not be necessary before referral for invasive electrophysiological study and/or catheter ablation 1. This is particularly appropriate for patients with:

• Drug-resistant or drug-intolerant symptoms 1
• Desire to be free of chronic drug therapy 1
• Severe symptoms such as syncope or dyspnea during palpitations 1

Common Pitfalls

Automatic ECG analysis systems are unreliable and commonly suggest incorrect arrhythmia diagnoses - always perform manual interpretation 1.

Wide QRS-complex tachycardia (>120 ms) requires differentiation from ventricular tachycardia. AVNRT with bundle branch block is possible but less common. If SVT cannot be proven easily, treat as ventricular tachycardia to avoid potentially fatal administration of calcium channel blockers for VT 1.

Physical examination during tachycardia usually does not lead to definitive diagnosis, though regular cannon A waves strongly suggest AVNRT 1, 3.